1. Field of the Invention
The present invention relates to a method for delivering a fine particle dispersion and a device for delivering a fine particle dispersion by use of a microflow-channel.
2. Description of the Related Art
In recent years, various research has been made into performing chemical reactions, unit operations or the like at a scale in the order of micrometers. In connection with this, methods and devices have been suggested for extraction, separation, mixing or particle formation, focussing on the short diffusion times.
Dispersions of fine particles having a specific gravity smaller than that of the liquid medium thereof and having a size in the order of microns are usually caused to flow in channels (or pipes) having a cross-sectional area in the order of centimeters or more.
However, there has not been suggested any method for causing a dispersion of fine particles having a specific gravity smaller than the liquid medium thereof and having a size in the order of microns to flow efficiently in channels having an inner cross-sectional area in the order of micrometers.
This is based on the following reason: when a dispersion of fine particles having a specific gravity smaller than the liquid medium thereof and having a size in the order of microns is caused to flow in channels having an inner cross-sectional area in the order of micrometers, problems of stagnation or clogging of flow arise since the specific gravity of the fine particles is smaller than that of the medium, and the fine particles float. In particular, it has been known that a fluid in a channel having an inner cross-sectional area in the order of micrometers flows in the state of laminar flow due to the inner cross-sectional area being small. However, with laminar flow, there is no stirring inside the fluid, and accordingly, fine particles which float while the fluid flows gradually gather together and are thus deposited on the upper surface of the inside of the channel. When this process further proceeds, the channel may become clogged up. When the microflow-channel becomes blocked up, it is very difficult to free the clogging, resulting in a more serious problem.
Hitherto, it has been considered that the effect of buoyant forces on fine particles in a channel having an inner cross-sectional area in the order of micrometers becomes relatively small (see, for example, “Kagaku Kogaku (Chemical Engineering)” vol. 66 No. 2 (2002)). However, it has been found that floatation of dispersed fine particles in the order of microns due to buoyant forces becomes problematic. This is because the situation of the channel relative to the buoyant force direction is not ordinarily considered.
On the other hand, when a dispersion of fine particles in the order of microns is caused to flow in a channel having a cross-sectional area in the order of centimeters, fine particles rarely float and deposit on the upper surface within the channel. It can be considered that although the floatation of the fine particles could physically occur, the fluid in the channel flows in a turbulent flow state, so that the fluid is agitated and the fine particles are not floated, and the deposition of the fine particles on the upper surface within the channel does not take place.
As methods for performing chemical reactions, unit operations or the like at a micrometer-scale as mentioned above, for example, a solvent-extracting method using a micro-fluid-system is suggested (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2002-361002). However, this is not a method for use in a system which contains fine particles.
The following methods have been proposed: a method of forming mono-dispersed metal nanometer-sized particles continuously; a method of growing crystals by use of the above nanometer-sized particles as nuclei; and a method of performing demineralization or removal of decomposition products continuously from a colloidal dispersion of nanometer-sized particles. However, the size of the metal nanometer-sized particles is described as 10 nm or less, and particles larger than this size are not described (see, for example, JP-A No. 2003-193119).
Further, a new method of using a micro-mixer to produce morphologically uniform micrometer-sized particles and nanometer-sized particles continuously, and the use of this method for encapsulating active substances, and particles produced by this method has been suggested. However, the size of the produced micrometer-sized particles is from about 1 to about 1000 μm (about 10 μm in the working examples). In cases where a dispersion containing particles having a smaller specific gravity than that of the liquid medium thereof is delivered in a microflow-channel, floatation or the like occurs within the channel, resulting in hindrance to fluid flow or clogging of the channel (see, for example, Published Japanese Translation of PCT International Publication for Patent Application No. 2003-500202).
In short, no method for delivering a dispersion of fine particles in the order of microns stably and without clogging the dispersion-delivering channel has been suggested. For example, a method of synthesizing fine particles and delivering a dispersion containing the synthesized fine particles in a microflow channel, is carried out. However, the size of the fine particles is limited to a size in the order of nanomicrons. In methods for producing particles or the like in the order of microns by use of a microreactor, the particles are discharged from the system immediately after the particles are produced. Thus, no method wherein a liquid containing the particles is delivered in a microflow-channel has been suggested.
As described above, a method for delivering a dispersion of fine particles in the order of microns stably without the problems of floatation or clogging, or a device therefor has not been suggested.